This invention relates to a heating apparatus which is effective for use as a heating means in a freeze-drying apparatus or a defreezing apparatus for food.
If food is freeze-dried, a product is obtained having the characteristic features of color, aroma, taste and vitamins of the raw material comparatively well preserved therein. When cold water, hot water or the like is later added to the freeze-dried material, the same is easily restored to its original state.
If freeze-drying is used for processing food, transportation or storage of the processed food is improved in efficiency. Accordingly, local or seasonal limitations of food production can be eliminated and the rationalization of the food industry can be carried out.
Additionally, the dried product can be preserved for long periods without requiring any special equipment, such as a refrigerator or the like. Cooking thereof is also simple, so that the same is useful for improvement in one's diet.
However, the freeze-drying of food requires that frozen food be dried by heat. It takes a long time, such as 6-10 hours, to dry the food when only a radiation heating apparatus, such as an electric heat-type heating apparatus or the like, is used.
In this respect, the inventors of the present invention have previously succeeded in inventing a heating apparatus including a microwave heating means using leakage-type waveguides having such a characteristic property that any material to be heated is heated from the inside thereof. The microwave heating means is combinedly used in the radiation heating apparatus so that the food to be heated may be heated from the inside and the outside thereof at the same time. Thus, the drying time required for obtaining the same freeze-dried food need only be about 2 hours.
However, the requirement of 2 hours for drying frozen food is still too much, and is unsuitable from the viewpoint of productivity. In view of this, as a result of further experimental research, the inventors have found that heating by a high frequency of 1000 KHz or so is effective in defreezing.
Namely, to explain in more detail, when a dielectric loss coefficient (tan .delta.) of water is measured by holding water as dielectric between electrode plates and a super-high frequency of 3 MHz is applied between the electrode plates, it is found to be 1,570.times.10.sup.-4. In the case of ice, it is so extremely small as 9.times.10.sup.-4. Accordingly, a super-high frequency heating apparatus requires much time for melting of ice and is not so effective.
Whereas, when the dielectric loss coefficient is measured in the case when a high frequency of 1000 KHz is applied between the electrode plates, it becomes as small as 400.times.10.sup.-4 for water; while it becomes as large as 1200.times.10.sup.-4 for ice.
Accordingly, if heated by the high frequency of 1000 KHz or so, a layer of ice covering the surface of food to be heated rapidly becomes molten, so that the drying time by heat can be shortened.
This invention has been made on the basis of this finding. It has been achieved that, according to this invention, using a high frequency heating means together with a radiation heating means and a microwave heating means quickens defreezing, and the necessary frozen food drying time is shortened to about 1 hour.
Next, this invention will be explained with reference to some embodying examples in which this invention is suitably applied to a heating apparatus for a vacuum freeze-drying apparatus for frozen food.